1. Field of the Invention
The present invention relates to an apparatus which includes a virtually imaged phased array (VIPA) to demultiplex a wavelength division multiplexed (WDM) light. More specifically, the present invention relates to an apparatus which includes a VIPA in combination with a wavelength splitter, such as a diffraction grating, to allow a wavelength division multiplexed light having a relatively large number of closely spaced wavelength components to be accurately demultiplexed.
2. Description of the Related Art
Wavelength division multiplexing is used in fiber optic communication systems to transfer a relatively large amount of data at a high speed. More specifically, a plurality of carriers, each modulated with information, is combined into a wavelength division multiplexed light. The wavelength division multiplexed light is then transmitted through a single optical fiber to a receiver. The receiver splits the wavelength division multiplexed light into the individual carriers, so that the individual carriers can be detected. In this manner, a communication system can transfer a relatively large amount of data over an optical fiber.
Therefore, the ability of the receiver to accurately split the wavelength division multiplexed light will greatly effect the performance of the communication system. For example, even if a large number of carriers can be combined into a wavelength division multiplexed light, such a wavelength division multiplexed light should not be transmitted if the receiver cannot accurately split the wavelength division multiplexed light. Accordingly, it is desirable for a receiver to include a high-precision wavelength splitter.
FIG. 1 is a diagram illustrating a conventional filter using a multiple-layer interference film, for use as a wavelength splitter. Referring now to FIG. 1, a multiple-layer interference film 20 is formed on a transparent substrate 22. Light 24, which must be parallel light, is incident on film 20 and then repeatedly reflected in film 20. Optical conditions determined by the characteristics of film 20 allow only a light 26 having wavelength .lambda.2 to pass therethrough. A light 28, which includes all light not meeting the optical conditions, does not pass through the film 20 and is reflected. Thus, a filter as illustrated in FIG. 1 is useful for splitting a wavelength division multiplexed light which includes only two carriers at different wavelengths, .lambda.1 and .lambda.2. Unfortunately, such a filter, by itself, cannot separate a wavelength division multiplexed light having more than two carriers.
FIG. 2 is a diagram illustrating a conventional Fabry-Perot interferometer for use as a wavelength splitter. Referring now to FIG. 2, high-reflectance reflecting films and 32 are parallel to each other. Light 34, which must be parallel light, is incident on reflecting film 30 and reflected many times between reflecting films 30 and 32. Light 36 of wavelength .lambda.2 that meets passage conditions determined by the characteristics of the Fabry-Perot interferometer passes through reflecting film 32. Light 38 of wavelength .lambda.1, which does not meet the passage conditions, is reflected. In this manner, light having two different wavelengths can be split into two different lights corresponding, respectively, to the two different wavelengths. Thus, as with the filter illustrated in FIG. 1, a conventional Fabry-Perot interferometer is useful for splitting a wavelength division multiplexed light which includes only two carriers at different wavelengths, .lambda.1 and .lambda.2. Unfortunately, such a Fabry-Perot interferometer cannot separate a wavelength division multiplexed light having more than two carriers.
FIG. 3 is a diagram illustrating a conventional Michelson interferometer for use as a wavelength splitter. Referring now to FIG. 3, parallel light 40 is incident on a half mirror 42 and split into a first light 44 and a second light 46 perpendicular to each other. A reflecting mirror 48 reflects first light 44 and a reflecting mirror 50 reflects second light 46. The distance between half mirror 42 and reflecting mirror 48, and the distance between half mirror 42 and reflecting mirror 50 indicate an optical path difference. Light reflected by reflecting mirror 48 is returned to half mirror 42 and interferes with light reflected by reflecting mirror 50 and returned to half mirror 42. As a result, lights 52 and 54 having wavelengths .lambda.1 and .lambda.2, respectively, are separated from each other. As with the filter illustrated in FIG. 1 and the Fabry-Perot interferometer illustrated in FIG. 2, the Michelson interferometer illustrated in FIG. 3 is useful for splitting a wavelength division multiplexed light which includes only two carriers at different wavelengths, .lambda.1 and .lambda.2. Unfortunately, such a Michelson interferometer cannot separate a wavelength division multiplexed light having more than two carriers.
It is possible to combine several filters, Fabry-Perot interferometers or Michelson interferometers into a giant array so that additional wavelength carriers can be split from a single wavelength division multiplexed light. However, such an array is expensive, inefficient and creates an undesireably large receiver.
A diffraction grating or an array waveguide grating is often used to split a wavelength division multiplexed light comprising two or more different wavelength carriers.
FIG. 4 is a diagram illustrating a conventional diffraction grating for splitting a wavelength division multiplexed light. Referring now to FIG. 4, a diffraction grating 56 has a grating surface 58. Parallel light 60 having a plurality of different wavelength carriers is incident on grating surface 58. The different wavelength carriers are reflected at each step of grating surface 58 and interfere with each other. As a result, carriers 62, 64 and 66 having different wavelengths are output from diffraction grating 56 at different angles, and are therefore separated from each.
Unfortunately, a diffraction grating outputs the different wavelength carriers at relatively small dispersion angles. As a result, it is difficult for a receiver to accurately receive the various carrier signals split by the diffraction grating. This problem is especially severe with a diffraction grating which splits a wavelength division multiplexed light having a large number of carriers with relatively close wavelengths. In this case, the angular dispersion produced by the diffraction grating will be extremely small, and typically .apprxeq.0.05 degree/nm.
In addition, a diffraction grating is influenced by the optical polarization of the incident light. Therefore, the polarization of the incident light can affect the performance of the diffraction grating. Also, the grating surface of a diffraction grating requires complex manufacturing processes to produce an accurate diffraction grating.
FIG. 5 is a diagram illustrating a conventional array waveguide grating for splitting a wavelength division multiplexed light. Referring now to FIG. 5, light comprising a plurality of different wavelength carriers is received through an entrance 68 and is divided through a number of waveguides 70. An optical exit 72 is at the end of each waveguide 70, so that an output light 74 is produced. Waveguides 70 are different in length from each other, and therefore provide optical paths of different lengths. Therefore, lights passing through waveguides 70 have different phase from each other and thereby interfere each other when they are output through exit 72. This interference causes lights having different wavelengths to be output in different directions.
In an array waveguide grating, the dispersion angle can be adjusted to some extent by properly configuring the waveguides. However, an array waveguide grating is influenced by temperature changes and other environmental factors. Therefore, temperature changes and environmental factors make it difficult to properly adjust the dispersion angle.